J. Org. Chem. 1983,48, 261-263 Inorganics) was recrystallized from hexane. Iron pentacarhnyl (Pressure Chemicals) was distilled under nitrogen in the dark. tertButy1 peroxide and hydroperoxide were commercial samples that were purified according to literature procedures." Di-n-hexyl peroxide [bp 65 OC (0.23torr); likSbp 58 OC (0.5 torr)], di-see-butyl peroxide [bp 43 "C (18 torr); lit.s bp 59 OC (50 torr)], n-hexyl hydroperoxide [bp 47 "C (2.3 torr); lit.'bp 42 OC (2 torr)], and see-butyl hydroperoxide [bp 45 OC (11torr); lit! bp 41 "C (11 torr)] were all prepared by following literature procedures. 'H N M R spectra were recorded on a Varian T-60 spectrometer, and GLC analyses were performed on a Varian Model 90P instrument equipped with a thermal-conductivity detector and a Hewlett-Packard Model 3380A electronic integrator. Unexceptional internal standard techniques were employed for quantitation. HPLC assays were carried out on a Waters Associates Model 6000A instrument equipped with a refractive index detector. Capillary GC/MS was carried out on a Hewlett-Packard Model 5985B instrument employing 50 m X 0.25 mm fused silica columns of Carbowax 20M or methylsilicone. Typical Reactions between Alkyl Peroxides and Hydroperoxides and Dicobalt Octacarbonyl and Iron Pentacarbonyl. In an inertatmosphere drybox dicobalt octacarbonyl (0.342 g, 1.00 mmol) was placed in a 40-mL centrifuge tube equipped with a Teflon-coated stirrer bar. The vessel was capped with a rubber septum and removed to the bench top. Benzene (5 mL) was added by syringe and the vessel cooled in a temperature bath to 6 8 OC. To this solution was subsequently added by syringe, slowly and with vigorous stirring, a solution of n-hexyl hydroperoxide (0.591 g, 5.00 mmol) in benzene (5 mL). After an appropriate time (seeTable II),the resulting mixture was fiitered under nitrogen and analyzed by HPLC on a p-Bondapak column by using methanol-water (3070). Reaction of Iron Pentacarbonyl with sec-Butyl Hydroperoxide. A solution of see-butyl hydroperoxide (451 mg, 5.00 mmol) in benzene (5 mL) was injected into a 40-mL centrifuge tube equipped with a Teflon-coated stirrer bar and capped with a rubber septum. The vessel was cooled ( 6 8 OC) before a solution of iron pentacarbonyl(196 mg, 1.00 mmol) in benzene was slowly added, accompanied by vigorous stirring. After a predetermined time, the contents of the vessel were filtered under nitrogen, and the product mixture was analyzed by HPLC on a p-Bondapak C18 column by using a methanol-water (3070) eluent. Reaction of tert-Butyl Hydroperoxide with Dicobalt Octacarbonyl in n -Heptane. To a vigorously stirred solution of dicobalt octacarbonyl (0.171 g, 0.500 mmol) in olefin-free nheptane (5 mL) at 0 OC was added by syringe a solution of tert-butyl hydroperoxide (0.901g, 10.0 mmol) in n-heptane. After 1 h the mixture was filtered under nitrogen. Analysis of the resulting solution by GC/MS revealed the presence of a number of solvent-derived producta, e.g., 2-heptanone, 4-heptanol, 3heptanone, and 4-heptanone, identified by a comparison of their respective mass spectra to literature spectra. In addition, an unknown compound with the empirical formula CllHzr02was observed. On the basis of ita characteristic mass spectrum [mle (relative intensity) 189 (M 1,0.2), 188 (M', L7), 131 (O.l), 115 (0.3), 99 (0.2), 98 (9.2), 73 (5.0), 57 (loo), 43 (24)], this material is tentatively formulated as the unsymmetrical peroxide tC4H900C7H15
261
Thermolysis of Dioxetanes: Activation Parameters for cis -/trans -3,4-Dialkyl-l,%-dioxetanes Alfons L. Baumstark,*' Tambra Dunams, Peter C. Roskamp, and Catherine E. Wilson Laboratory for MBS, Department of Chemistry, Georgia State University, Atlanta, Georgia 30303 Received July 7,1982
The thermal decomposition of alkyl-substituted 1,2dioxetanes has been shown2 to produce two carbonyl fragments, one of which may be produced in an excited state (high yields of excited triplets). Historically, two mechanistic extremes have been proposed2 to describe the thermal decomposition of alkyldi~xetanes:~ (a) diradical
and (b) concerted (Scheme I). Evidence has accumulated in favor of a diradical mechanism. Group additivity calculations have been employed4 to predict activation parameters based on the thermochemistry of the dioxetane and the postulated diradical intermediate. Experimental results (lack of solvent effect: insensitivity of E, to phenyl for methyl substitution: lack of deuterium isotope effect,' lack of additional ring-strain effect0 on E,) have been interpreted t o be consistent with a diradical-like mechanism. Recent resultsg have shown that substituent effects influence the activation parameters of the thermolysis of alkyldioxetanes in unexpected ways. Despite the considerable interest in substituent effects on dioxetane activation parameters, there has been no characterization of cis/trans pairs of dioxetanes (although several pairs have been synthesized).2J0 We report the characterization of three pairs of cis-/trans-3,4-dialkyl-l,2-dioxetanes (1-6).
1, R, = Et; R,
Et; R, = H R, = Et = n-propyl; R, = n-propyl; R, = H = n-propyl; R, = H; R, = n-propyl = n-butyl; R, = n-butyl; R, = H = n-butyl; R, = H; R, = n-butyl
2, R, = Et; R, = H;
3, R, 4, R, 5, R, 6,R,
Results and Discussion Dioxetanes 1-6 were prepared in low yield by closure of the corresponding bromo hydroperoxides with base at
+
(1) Fellow of the Camille and Henry Dreyfus Foundation, 1981-1986. (2)For reviews see: (a) Wilson, T. Znt. Reu. Sci.: Phys. Chem., Ser. T w o 1976,9,265.(b) Adam, W. Adu. Heterocycl. Chem. 1977,21,437. (c) Horn, K.A.; Koo, J.; Schmidt, S. P.; Schuster, G. B. Mol Photochem. 1978,9 (l),1. (3)The electron-transfer mechanism(8) of chemiluminescent decomAcknowledgment. We thank Dr. Robert H. Schwartz, position (high yields of excited singlets) does not occur readily with alkyl-substituted 1,2-dioxetanes. who carried out some of the preliminary experiments de(4)ONeal, H. E.;Richardson, W. H. J.Am. Chem. SOC.1970,92,6553; scribed in this work. correction, 1971,93,1828. Registry No. Di-n-hexyl peroxide, 3903-89-7; di-see-butyl (5)Wileon. T.: Landis, M. E.: Baumstark, A. L.: Bartlett, P. D. J.Am. Chem. SOC.1973,95,4765. peroxide, 4715-28-0; di-tert-butyl peroxide, 110-05-4; n-hexyl (6)Richardson, W. H.; Burns, J. H.; Price, M. E.; Crawford, R.; Foster, hydroperoxide, 4312-76-9;see-butyl hydroperoxide, 13020-06-9; 1978,100,7596 and M.; Slusser, P.; Andergg, J. H. J. Am. Chem. SOC. tert-butyl hydroperoxide, 75-91-2; Fe(CO)5,13463-40-6;CO~(CO)~, references therein. 10210-68-1. (7)Koo, J.; Schuster, G. B. J. Am. Chem. SOC.1977,99,5403. (8)Wilson, T.; Golan, D. E.; Scott, M. S.; Baumstark, A. L. J . Am. Chem. SOC.1976,98,1086. (9)(a) Baumstark, A.L.; Wilson, C. E. Tetrahedron Lett. 1981,4363. (4)Bartlett, P.D.; McBride, J. M.J. Am. Chem. SOC. 1966,87,1727. (b) Bechara, E. J. H.; Wilson, T. J. Org. Chem. 1980,45, 5261. (c) (5)Williams, H.R.;Mosher, H. S. J. Am. Chem. SOC. 1964,76,2984, Lechtken, P.; Reiasenweber, G.; Grubmueller, P. Zbid. Tetrahedron Lett. 2987. 1977,2881. (d) Baumstark, A. L.; Wilson, C. E. Zbid. Tetrahedron Lett. 1965, (6)Welch, F.; Williams, H. R.; Mosher, H. S. J.Am. Chem. SOC. 1979,2569. 77, 551. (IO) (a) White, E. H.; Wildes, P. D.; Wiecko, J.; Doshan, H.; Wei, C. (7)Cookson, P. G.; Davies, A. G.; Roberta,B. P. J.Chem. SOC.,Chem. C. J. Am. Chem. SOC. 1973,95,7050.(b) Wilson, T.; Schaap, A. P. Zbid. Commun. 1976,1022. 1971, 93,4126. (c) Schaap, A. P.; Bartlett, P. D. Zbid. 1970,92,6055. (8)Kochi, J. K. J. Am. Chem. SOC. 1962,84,1193.
0022-326318311948-0261$01.50/00 1983 American Chemical Society
262
J. Org. Chem., Vol. 48, No. 2, 1983
Notes
Scheme I
- . i ' I'
0-0
diradical
\
'e
--
/
I
z,--
10-
concerted An asterisk denotes an excited state.
i
Table I. Activation Parameters of the Thermal Decomposition of 1-8 in Xylenes dioxetane AS', cis trans E,, kcal/mol log A k(60 "C), s - ' eu 24.5 t 24.9 * 24.5 k 3 25.0 * 4 24.6 * 5 25.1 k 6 tetrameth- 27.3 * yl-l,a-dioxetane 1"
2
0.3 0.3 0.3 0.3 0.3 0.3 0.3
13.1 13.1 13.1 13.2 12.9 13.1 13.8
1.15 x l o - ' 5.9 x 1.1 x l o e 3 6.8 X 5.8 x 4.6 x 8.0 x
-0.7 -0.8 -0.7 -0.2 -1.7 -0.6 2.4
See ref 10. 95%confidence limit. In good agreement with the results reported in ref 8 and 9c.
low temperature. The cis-1,2-dioxetanes were prepared (via two steps) from the corresponding cis-alkenes while the trans-1,2-dioxetanes were prepared from the transalkenes.'OJ1 'H NMR spectra of the dioxetanes indicated that the cis compounds contained no detectable quantities of the trans compounds and vice versa. NMR spectroscopy showed that the thermolysis of 1-6 produced the expected cleavage products. The IH NMR signals for the ring protons for the cisdioxetanes (1, 3, 5) were observed a t 6 -5.2 while those for the trans-dioxetanes (2,4,6) were observed a t 6 -5.0 in CC1,. This is in agreement with the reported data for the ring protons of cis-/trans-3,4-diethoxy-1,2-dioxetane'Oc (chemical shift difference = 0.3 in CFC1,) and for cis-/ trans-3,4-dimethyl-1,2-dio~etane'~~ (chemical shift difference = 0.4; CCl., vs. benzene). The chemical shift for the ring protons of cis-3,4-disubstituted 1,2-dioxetanes is downfield from that of the corresponding trans-dioxetanes. The rates of the thermal decomposition of dioxetanes 1-6 were monitored by the decay of chemiluminescence intensity in aerated xylenes or benzene with or without added fluorescers. The rates of thermolysis were cleanly first order for a t least 3 half-lives and showed little or no dependence on the type or amount of added fluorescer. For all three pairs, over the temperature range studied, the cis compound decomposed a t a rate faster than the trans isomer. The first-order rate constants were determined over at least a 50 OC range. The Arrhenius plot for the thermal decomposition of 3 and 4 is shown in Figure 1. The activation parameter data (determined by the Arrhenius method) for 1-6 are summarized in Table I. Without the addition of fluorescers (fluorescent dyes), the thermal decomposition of dioxetanes 1-6 were only (11) (a) Bartlett, P. D.; Landis, M. E.; Shapiro, M. J. J. Org. Chem. 1977,42,1661. (b) Koo, J.; Schuster, G. B. J. Am. Chem. SOC.1977,99, 5403.
4. I 2 80
2 90
3 00
3 10
3 20
IOOO~T'K
Figure 1. Arrhenius plota for the thermal decomposition of 3 ( 0 ,DBA; 0, DPA) and 4 (A,DBA; A, DPA). Table 11. Variation in E, for &/trans-Dioxetanes
1-2 3-4 5-6 cis/trans-3,4-diethoxy1,a-dioxetane a
95%confidence limits.
1.9 1.6 1.2 1.4
0.4 i 0.6" 0.5 ?; 0.6 0.5 * 0.6
At 50 'C; see ref 1Oc.
weakly chemiluminescent. Addition of low concentrations of dibromoanthracene (DBA) or diphenylanthracene (DPA) greatly increased the intensity of chemiluminescence without increasing the rate of the thermolysis of the dioxetanes. The yields of excited carbonyl products directly produced by the thermal decomposition of the dioxetanes were determined by the DBA/DPA method.laS8 The thermal decomposition of dioxetanes 1-6 produced approximately 10% yields of excited triplet carbonyls and very low yields (
